Fixed wrong default argument of `config`.

It caused `IOError [Errno 21] Is a direcotory`
when user runs it without -c option.

Here's full traceback:
```
Traceback (most recent call last):
  File "/home/hirokiky/dev/doorman/env/bin/doorman", line 9, in <module>
    load_entry_point('doorman==0.4.0', 'console_scripts', 'doorman')()
  File "/home/hirokiky/dev/doorman/doorman/doorman/main.py", line 44, in main
    doorman = Doorman(args.status, os.path.abspath(args.config_file))
  File "/home/hirokiky/dev/doorman/doorman/doorman/doorman.py", line 64, in __init__
    self.__config = DoormanConfig(config_file).parseYAML()
  File "/home/hirokiky/dev/doorman/doorman/doorman/doorman.py", line 35, in parseYAML
    with open(self.__config_file, "r") as f:
IOError: [Errno 21] Is a directory: '/home/hirokiky/.config/doorman'
```
I know it doesnt contain any additions of tests. but I couldn't find good place to write (tests for command line options or some).